1. Field of the Invention
The present invention relates to a magnetic recording medium and a method of producing it which is suitable for use in a hard-disk unit.
2. Prior Art
In general, in order to raise the recording density of the magnetic recording medium to be used in the hard-disk unit, it is preferable that a gap (or space) between the magnetic recording medium and magnetic head is relatively small. For this reason, the surface of the magnetic recording medium must be formed smoothly. However, if the surface of the magnetic recording medium is extremely smooth, there occurs a head-absorption phenomenon in which the magnetic head is absorbed toward the magnetic recording medium in its operation, which may cause a head crash. In order to avoid such phenomenon, micro-irregularities are formed on the surface of the magnetic recording medium.
As the conventional methods of forming the micro-irregularities on the surface of the magnetic recording medium, there are two methods as follows.
(1) In the first method (which is disclosed in Japanese Patent Laid-Open Publication No. 1-165025), the anodic oxidation process is effected on Al or Al alloy substrate (i.e., aluminium substrate) so as to form the anodic-oxide film (e.g., alumite film). By use of the so-called "cell-pore structure" of the alumite film, non-magnetic material is filled into each cell-pore. Then, by effecting the etching process on the alumite film, the non-magnetic material is protruded from the surface of the alumite film, thus forming the micro-irregularities. PA1 (2) In the second method (which is disclosed in Japanese Patent Laid-Open Publication No. 2-58729), particulates at relatively low density are adhered on the aluminum substrate on which Ni-P metal-plating film is formed, thus forming the micro-irregularities.
However, the above-mentioned, conventional methods of producing the magnetic recording media suffer from the following drawbacks. The foregoing first method is effective when avoiding the head-absorption phenomenon. However, in the magnetic recording medium which is produced according to the first method, the micro-irregularities must be formed at relatively high density, and as illustrated in FIG. 1, a contact area formed between a magnetic recording medium 10a and a head slider 11 must be relatively large. For these reasons, the first method is not so effective when reducing frictional coefficient .mu. between the magnetic head and magnetic recording medium 10a in Contact-Start-Stop (i.e., CSS) operations. In the CSS testing operation, the magnetic head is forced to be in contact with the magnetic recording medium. The characteristics of the magnetic recording medium under the CSS operations are called "CSS characteristics". Normally, the frictional coefficient .mu. is required to be lower than "0.5" after performing the CSS operations by twenty thousand times. However, in the magnetic recording medium which is produced according to the first method, it is difficult to satisfy such requirement.
In the second method, it is difficult to control the sizes and density of the micro-irregularities to be formed on the magnetic recording medium.